Dropbox API Error when downloading image to raspi

This 'not_found' error indicates that "There is nothing at the given path". That is referring to the remote path on Dropbox for the file you requested to download. 

For the 'files_download_to_file' method, that's the second parameter, to which you are supplying the path "/picked_image1981016614414776853.jpg" according to this code. 

Are you sure that's the correct path? Note that if that access token is for a "Full Dropbox" app, the path should be the full path to the file, relative to the Dropbox folder, including any/all parent folders. If the access token is for an "app folder" app, the path should only be relative to the app folder itself.